The present invention relates to a method for the preparation of an organopolysiloxane or, more particularly, to a method for the preparation of an organopolysiloxane containing tetrafunctional siloxane units of the formula SiO.sub.2, of which the content of the SiO.sub.2 units relative to other types of the siloxane units can be well controlled to be the same as in the mixture of the starting low-molecular organosilicon compounds, without the addition of any ordinary acidic or alkaline material as a catalyst and an alcohol and further without undertaking troublesome disposal treatments of waste liquids containing acids and alcohols.
As is known, organopolysiloxanes composed of monofunctional siloxane units represented by the general formula R.sub.3 SiO.sub.1/2, in which R is a hydrogen atom or a monovalent hydrocarbon group, referred to as the M units hereinbelow, and the tetrafunctional siloxane units of the formula SiO.sub.2, referred to as the Q units hereinbelow, which soluble in various organic solvents such as benzene, are widely used in many applications in the art of silicones. A typical method in the prior art for the preparation of such an organopolysiloxane composed of the M and Q units it that a water-soluble basic silicate such as water glass, sodium orthosilicate and the like is converted into an oilgomer of silicic acid by the addition of a strong acid such as hydrochloric and sulfuric acids followed by the reaction with a triorgano monochlorosilane according to the disclosure in U.S. Pat. No. 2,676,182 and U.S. Pat. No. 2,814,601.
This method, however, has several problems, viz., the molecular weight distribution of the organopolysiloxane product can hardly be controlled because of the instability of the oilgomeric organopolysiloxane as the intermediate, the molar proportion of the M units and Q units in the organopolysiloxane product may sometimes be deviated from the target value as desired notwithstanding careful formulation of the starting materials, and the waste water discharged from the process must be disposed with large costs because it contains a large amount of the acid used for the neutralization as well as the hydrochloric acid produced as a by-product from the trialkyl monochlorosilane as well as a large amount of the alcohol added to the reaction mixture with an object of stabilization of the reaction mixture. In particular, hydrochloric acid or hydrogen chloride gas therefrom is not only harmful to the human body and has a very unpleasant irritating odor but also is extremely corrosive against metal-made parts in the manufacturing apparatuses, pipings and accessary equipments necessitating a great expense for the apparatuses and equipments made of corrosion-resistant but very expensive materials as well as maintenance thereof with a great deal of labor and material.
Alternatively, different methods are known for the preparation of the organopolysiloxanes composed of the M and Q units including the method disclosed in U.S. Pat. No. 2,857,356 in which an alkyl silicate and a trialkyl monochlorosilane are cohydrolyzed in the presence of hydrochloric acid and a method disclosed in Japanese Patent Kokai No. 61-195129 in which an alkyl silicate or a partial hydrolysis product thereof is added dropwise to a hexaalkyl disiloxane or a trialkyl monochlorosilane in the presence of aqueous hydrochloric acid. These methods are more advantageous than the first mentioned method in respect of the relatively easy control of the molar proportion of the M and Q units and the molecular weight distribution in the product. On the contrary, these methods are very disadvantageous in respect of environmental pollution and waste disposal because the reaction mixture is admixed with a large amount of hydrochloric acid or a large amount of hydrogen chloride is produced in the course of the reaction and a large amount of an alcohol is produced in the reaction mixture from the starting materials or the reaction mixture must be admixed with an alcohol so that the problems relative to the waste disposal and prevention of damages by the hydrochloric acid are as serious as in the first mentioned method.
Thus, each of the above described prior art methods has problems and disadvantages that a very expensive process of waste disposal is indispendable, that the efficiency of production is relatively low with a low yield of the organopolysiloxane product per unit volume of the reaction vessel as a consequence of a large volume of organic solvents as an auxiliary material in the hydrolysis, that difficulties are encountered in the control of the reaction when the desired organopolysiloxane product contains the M units in a relatively large proportion or, in particular, with the molar ratio of M units/Q units of 2 or larger, than an organopolysiloxane having a specific target values of the M units/Q units molar ratio and the molecular weight distribution can hardly be obtained, and that organopolysiloxane resins having silanol groups and alkoxy groups can hardly be manufactured with good reproducibility.